Gmail's scheduling feature could be used to create a series of timed, progressive emails aligned to a specific topic or theme. For example, you could create a series of four emails that contain directions on how to use features of Google Forms. The series might start with the basics of using Google Forms and then each subsequent email would build upon that. Each email could contain a written overview, a video overview, and a "do now" practice activity.
Watch this short video to see a demonstration of how to create a professional development series in Gmail.
Applications for Education
In the video above I used the schedule send function in Gmail to create a professional development series for teachers. The same concept could be applied to creating a mini-course for students to complete over the summer. If you teach an AP course that requires students to complete some assigned reading over the summer, you could create a series of scheduled emails to check in with them and have them reply to a couple of questions. Of course, you might also do that in Google Classroom.